1. Field of Invention
This invention relates generally to electromagnetic flowmeters, and in particular to a flowmeter which includes a lined, non-magnetic spool serving as the conduit for the fluid being metered, and a pair of electrode assemblies encapsulated within an insulating layer molded against the inner surface of the spool to provide a capacitance sensor for measuring fluids.
2. Prior Art
Magnetic flowmeters such as those disclosed in U.S. Pat. Nos. 3,695,104; 3,824,856; 3,783,687 and 3,965,738, are especially adapted to measure the volumetric flow rates of fluids which present difficult handling problems, such as corrosive acids, sewage and slurries. Because the instrument is free of flow obstructions, it does not tend to plug or foul.
In a magnetic flowmeter, an electromagnetic field is generated whose lines of flux are mutually perpendicular to the longitudinal axis of the flow tube through which the fluid to be metered is conducted and to the transverse axis along which the electrodes are located at diametrically-opposed positions with respect to the tube. The operating principles are based on Faraday's law of induction which states that the voltage induced across any conductor as it moves at right angles through a magnetic field will be proportional to the velocity of that conductor. Because the metered fluid effectively constitutes a series of fluid conductors moving through the magnetic field; the more rapid the rate of flow, the greater the instantaneous value of the voltage established at the electrodes.
The typical commercially-available magnetic flowmeter is provided with mounting flanges at either end thereof. The meter is interposed between the upstream and downstream pipes of a fluid line, each pipe having an end flange. The mounting flanges on the meter are bolted to the flanges of line pipes. It is, of course, essential that the circle of bolt holes on the mounting flanges of the meter match those on the pipe flanges.
In a magnetic flowmeter, the flow tube is subjected to the same fluid pressure as the line pipes. The flow tube must therefore be of a material and of a thickness sufficient to withstand this pressure, even though the strength of the flow tube is unrelated to its measuring function. This design factor contributes significantly to the cost of a standard meter. Existing meters are made up of components that must be assembled, and are generally of substantial size and weight and quite expensive to manufacture.
In order to provide a compact and readily installable electromagnetic flowmeter whose weight and dimensions are substantially smaller than existing types, the above-identified related patent applications and patents disclose highly compact flangeless flowmeters which, despite their reduced volume and weight, are capable of withstanding high fluid pressures, the flowmeters operating efficiently and reliably to accurately measure flow rates.
The main concern of the present invention is with the problem of galvanic and slurry noises which are exhibited by meters having electrodes in contact with the fluid being metered; another concern is the measurement of fluids whose conductivity is so low that it approaches that of a dielectric.
In the Appel U.S. Pat. No. 4,019,386 and in my prior patent 4,098,118, instead of small area measuring electrodes in direct contact with the fluid being metered, use is made of electrode assemblies encapsulated in insulation material. Each assembly is formed by a measuring electrode having a large area behind which is a driven shielding electrode of even greater area, the measuring electrodes being isolated from the fluid by a layer of insulation. Each measuring electrode forms one plate of a capacitor whose dielectric is the insulation layer and whose other plate is the fluid, the electrodes acting as a capacitance sensor to detect the voltage induced in the fluid.
A capacitance electrode sensor of this type obviates slurry and galvanic noise problems and is not subject to leakage. Also among the advantages of a capacitance sensor over contact electrodes in a magnetic flowmeter are that the conductivity range of the fluid to be metered may extend down to as low as 0.1 .mu.S/cm or less, and one may use ordinary metals for the electrodes rather than special materials capable of withstanding the adverse effects of corrosive or abrasive fluids in contact with the electrodes.
In constructing a flowmeter of the type disclosed in the Appel et al. patent, an electrode assembly package constituted by a curved measuring electrode plate and a curved shielding electrode plate is placed against the curved inner surface of the flow tube. Then a non-conductive plastic potting compound is cast into the flow tube to encapsulate the electrode assembly package and at the same time to define a uniform flow passage through which the fluid is conveyed to intercept a magnetic field and thereby induce a signal in the capacitance measuring elec- trodes.
With a unitized electrode assembly of the Appel et al. type, it often becomes difficult to match the curvature of the assembly to the curvature of the tube to a degree necessary to eliminate air spaces in the final encapsulation and thereby minimize the possibility of electrode motion. Such electrode displacement gives rise to noise in the meter output signal and has other adverse effects. Also a unitized electrode package, when its thickness is significant as is often the case, introduces widely differing section thickness in the final encapsulant, which in turn causes uneven shrinkage during the curing process and undesirable internal stresses in the potting compound.